A well-known and widely used type of vehicle safety seat belt retractor, the emergency locking type, allows the belt to be unwound except when the vehicle is abruptly decelerated or accelerated, such as occurs in a collision. Numerous mechanisms that detect the rate of change in vehicle speed (positive or negative acceleration) by an inertia detecting device and lock the belt reel when the acceleration exceeds a preset level are known in the prior art. Such mechanisms may respond directly to acceleration of the vehicle in any direction, e.g., in mechanisms that use pendulums, tilting weights or rolling or sliding masses, or to rapid acceleration of the belt reel, e.g., mechanisms that include an inertia wheel that normally rotates with the belt reel but lags behind the reel due to rapid unwinding (rotary acceleration) of the belt. Emergency locking retractors are popular because they allow the user to change position or loosen the belt for comfort at will.
The ability of the belt to unwind from an emergency locking retractor against the relatively small force of the winding spring, which force is kept small so that the belt is not pulled uncomfortably tightly against the wearer, presents a problem when a conventional emergency locking retractor is used to secure child safety seat to the vehicle seat. The problem is that the safety seat can move forward or sideways due to braking, turning or other normal maneuvers and motions of the vehicle. Moreover, a collision is often preceded by a sharp turn or hard braking that is intended to avert the collision or that itself results in a collision (e.g., loss of control or a skid). The child safety seat may move in these situations, with the result that it is not tightly held on the vehicle seat and can, along with its occupant, be thrown about when a collision occurs.
The above-described problem is well known and various solutions have been proposed, among them manual locking devices which require the user to push a button or move a lever to change the mode of operation of the reel locking mechanism from emergency to automatic.
Matsui U.S. Pat. No. 4,436,255 (Mar. 13, 1984) describes and illustrates a seat belt retractor that automatically changes back and forth between an emergency-locking mode and an automatic-locking mode in accordance with the winding and unwinding of the belt, the conversion being caused by a cam wheel that rotates through a small fraction of a revolution for each revolution of the belt reel. The cam wheel moves a spring between positions holding a locking pawl in engagement with a locking ratchet wheel of the retractor and a position out of engagement with the ratchet wheel, the latter position establishing the emergency locking configuration of the retractor. The spring continually engages the cam wheel and is subject to friction and wear whenever the belt is being wound onto or unwound from the reel.
The present inventor proposed in Takada U.S. Pat. No. 4,565,338 (Jan. 21, 1986) a retractor having a mechanism for activating an automatic locking mode in response to withdrawing nearly all of the belt from the belt reel. The retractor of that patent includes a pivotable actuator engageable with a reel-locking pawl in the locked and unlocked conditions through a lost motion coupling, an over-center spring associated with the retractor for biassing it toward the respective engagements and a reduction mechanism driven by the reel and having an output member having two circumferentially spaced-apart lugs for moving the actuator between its two positions. The design of that retractor is such that the automatic locking mechanism is activated only after nearly all of the belt is unwound from the reel; the lug that activates automatic locking cannot be rotated in the belt-unwinding direction past the position where activation occurs, lest in rewinding that same lug will deactivate the automatic locking mechanism. This means that activation occurs only after a length of the belt greater than that required to hold the child safety seat has been withdrawn. Therefore, if the user pulls enough of the belt out to fasten the safety seat in place but not enough to activate the automatic locking mode, the safety seat will not be properly secured to the vehicle seat.
Yamamoto et al. U.S. Pat. No. 4,597,546 (July 1, 1986) has a mechanism similar to that of Takada U.S. Pat. No. 4,565,338 (discussed immediately above) and presents the same problem.